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Abstract

Current models of shallow rocky community organization and stability in the
northwestern Atlantic (NWA) emphasize kelps and their vulnerability to grazers and
other mortality agents. This paradigm may overshadow the possible contribution of other
groups of less studied seaweeds with overlapping distribution to ecosystem resilience.
The annual, brown seaweed Desmarestia viridis is one of only a few species of fleshy
seaweeds commonly found in urchin barrens in the NW A. The exceptional ability of
D. viridis sporophytes to produce and store sulfuric acid (H₂S0₄) in intracellular vacuoles
makes the species a compelling model for studies of controls and importance of acid
production in seaweeds at the individual, population, and community levels. This research
used laboratory experiments and surveys of individuals and populations throughout an
entire growth season (February to October 2011) at two subtidal sites on the southeastern
tip of Newfoundland (Canada) to determine controls of acid production in, as well as the
functional importance and persistence of, D. viridis sporophytes. Results showed that
light, grazing, and epibionts have no perceptible effects on intracellular acidity, whereas
temperature and wave action exert strong, synergistic effects. Mortality rates and sea
temperature from March to late June were relatively low, whereas the onset of increasing
mortality in mid-August coincided with marked increases in sea temperature. The quick
development of "Desmarestia beds" in urchin barrens created biological structure for
major recruitment pulses in characteristic invertebrate and fish assemblages. These
findings provide novel insights into the ecological and evolutionary causes and
consequences of acid production in Desmarestiales, while elevating the importance of
D. viridis as a foundation species in urchin barrens in the NWA.